This 3-year study involves a continuing survey of reptile populations throughout Colombia, with emphasis on contrasting ecological regions. The frequency and type of malaria parasites are determined for each region. In the final third year of the study, field personnel will repetitively sample certain selected lizard communities carrying Plasmodium infections and additionally characterized by relatively easy access and sufficiently high population density to permit basic demographic observations: population structure at each season, longevity and turnover rate, reproductive seasonality. This will permit testing the theory that malarial relapses coincide with host reproductive seasons. Parasite species showing promise as experimental organisms will be studied in the laboratory to determine host and parasite adaptability and the course of natural and induced infections. Information is being gathered simultaneously to prepare a field guide to the lizards of Colombia.